By 803056
Date 04-14-2008 18:02
Edited 04-14-2008 18:36
Hello Giovanni;
How are you?
We're starting to see the early signs of spring here in New England. A welcome sight indeed.
As you indicate, the need to grind to bright metal is based on the level of quality required. I equate it to the mill scale found on any hot rolled steel material. In most cases, the oxide associated with air carbon arc gouging is not as severe or as thick as that found on hot rolled material, but never the less, it should be removed when the highest quality welds are necessary. The amount of carbon introduced is minimal, if any, when the process is carried out properly. Any carbon introduced into the HAZ is by diffusion, which is time @ temperature dependent. The length of time at which the temperature is high enough to promote diffusion of the carbon into the HAZ is minimal. Any hardened HAZ associated with air carbon arc gouging is most likely due to the rapid cooling from the austenizing temperatures attained while gouging. Even then, the depth of the HAZ is akin to that developed as a result of oxy-fuel cutting, no better or no worse.
In my opinion, any welding performed after gouging will temper the HAZ. Any increase in hardness or increase in the depth of the HAZ is a result of the slower travel speeds and higher heat input and rapid cooling associated with the welding operation rather than the gouging operation.
With regards to crack removal, care must be exercised to prevent the crack from traveling. I found that in ductile base metals, gouging from beyond the crack tip toward the center of the crack, stopping, and gouging from beyond the opposite end of the crack toward the center, minimized the growth of the crack. Failure to do so is a recipe for disaster.
I had a foreman ignore my recommendations on one project (this was when I still used a welding shield instead of a computer to earn my living). They attempted to gouge a crack without regard to my recommendation and chased the crack right around a vessel. I did get the overtime pay to complete the repair. Still, the foreman faulted me for not objecting more strenuously. However, the same foreman stated, "I pay you from the neck down, not the neck up!"
I learned at an early age to never call the foreman an idiot or to intentional make him look stupid. That's "job suicide." Most foreman were able do that without my help.
As for the root cause of lamination; I have to agree with my friend Giovanni on this one. The root cause of a lamination, seam, pipe, or scab originates in the ingot from which the hot rolled product is rolled. Killed steels use deoxidizers to minimize the formation of carbon dioxide (oxygen combining with free carbon) in the ingot as the liquid metal solidifies. In the event carbon dioxide does form (and it will in rimmed steels, and to a lesser extent in semi killed steels as well as killed steels), the gas can be entrapped in the ingot just as gas can be entrapped in a weld. As such, it manifests itself as a pore, only in an ingot the pore can be very large. If it isn't completely removed, the pore (gas pocket) will be flattened and elongated by the subsequent rolling operation. Since the top center of the ingot is last to solidify, the bulk (not all) of the porosity will be found in the center of the upper portions of the ingot, the lamination is most often found toward the center of the thickness of the hot rolled plate, center of the web of a beam, or center of a bar.
Likewise, dross, oxides, low melting point constituents (LMPC) such as phosphorus, lead, excess copper, sulfur, etc. tend to segregate toward the center of the top portion of the ingot because that is the last region of the ingot to solidify. That portion of the ingot is supposed to be removed, but some of the objectionable material may remain in the portion of the ingot that is rolled into blooms, billets, or slabs. Subsequently, they will occur in the final hot rolled products as segregation's (also know as banding) which can cause welding issues during fabrication. The worst cases usually occur in thick sections where the rolling operation fails to "homogenize" the LMPCs and oxide stringers. They are less problematic in the thinner materials that are subjected to more "mixing" or "homogenization" by repeated rolling operations. That isn't to say they are never a problem with thin materials, just less so.
Good to read your responses. I always look forward to your well formulated opinions.
Best regards - Al